1. Field of the Invention
The invention relates to a method and system for the pre-processing and post-processing of audio or speech digital signals in order to maintain the quality of the signal at the highest possible level when there is a high noise level.
There is a major need to provide high-quality encoding and decoding systems, especially in the digital audio field. In applications such as videoconferencing, video telephony and multimedia applications, the audio signals need to be transmitted with high fidelity whatever the characteristics of the transmission channels.
2. Description of the Prior Art
In the system described in the patent application FR 2 815 492, also filed by the author of the present application and entitled “Radio broadcasting system and method providing continuity of service,” the discrete complex values of the information cells which normally convey QAM digital information (a limited number of permitted values generally equal to a power of two) are replaced by continuous values representing analog samples of the original or pre-processed audio signal. The quality of the audio signals at output is practically constant whatever the precise configuration of the channel.
The quality, even in frequency-selective channels, is similar to the quality of amplitude modulation (AM) in a Gaussian type channel with a broadcast power that may be less than 1/10 of the power needed for true amplitude modulation, because no continuous carrier is transmitted.
This is due to the fact that the receiver carries out a (complex) gain compensation of the channel at each frequency and each point in time in such a way that, in theory, there is no variation of gain in the samples received.
It can be observed that the gain compensation especially has the following effect: in every case, the noise added to the samples received after the modulation has a flat frequency spectrum.
The noise is Gaussian with a simple channel and is varyingly impulsive with frequency-selective channels (channels comprising multiple propagation paths).
The patent application WO 98/06090 discloses a system and method implementing a non-linear transform on the signal in order to augment the perceptible quality of the audio or phonic signal. The non-linear transform is applied to each of the components Sk of the spectrum obtained after a windowing and Fourier transform step.